At the present time, thermal transfer recording is widely used as a simple printing method. The thermal transfer recording can simply form various images, and thus is utilized in printing wherein the number of prints may be relatively small, for example, the preparation of ID cards, such as identification cards, photographs for business, or printers of personal computers or video printers. When a full-color halftone image, such as a photograph-like image of a face, is preferred, the thermal transfer sheet used is such that, for example, various colorant layers of yellow, magenta, and cyan (and, in addition, optionally black) are provided as ink layers in a large number in a face serial manner on a continuous substrate film. Such thermal transfer sheets are classified roughly into thermal transfer sheets of the so-called “melt transfer (ink transfer)” type wherein the colorant layer is melted and softened upon heating and as such is transferred onto an object, that is, an image-receiving sheet, and thermal transfer sheets of the so-called “sublimation (dye transfer)” type wherein, upon heating, a dye contained in the colorant layer is sublimated to permit the dye to migrate onto the image-receiving sheet.
When the above thermal transfer sheet is used, for example, to prepare ID cards, such as identification cards, the melt transfer type is advantageous in that line images, such as letters or numeric characters, can be easily formed, but on the other hand, the fastness property, particularly abrasion resistance, is disadvantageously poor. On the other hand, the sublimation type is suitable for the formation of halftone images, such as photograph-like images of a face. Unlike conventional printing inks, however, no vehicle is used. Therefore, the formed images are poor in fastness properties such as abrasion resistance, and, in addition, when brought into contact, for example, with plasticizer-containing card cases, file sheets, erasers made of plastics or the like, disadvantageously cause migration of dyes onto them. Further, the formed images have poor chemical resistance, solvent resistance and other properties and hence cause blurring or other unfavorable phenomena. For this reason, an attempt to further transfer a protective layer on the formed image from a protective layer transfer sheet has been made to further impart improved fastness properties, such as abrasion resistance, chemical resistance, and solvent resistance, to the formed images. For example, a protective layer transfer sheet comprising a substrate film, a transparent resin layer releasably provided on the substrate film, and a heat-sensitive adhesive layer provided on the transparent resin layer is used to transfer and stack a transparent resin layer on an object with an image formed thereon through the heat-sensitive adhesive layer.
Since, however, the heat-sensitive adhesive layer in the conventional protective layer transfer sheet consists of a resin alone, the coefficient of friction between the heat-sensitive adhesive layer and an object such as an image-receiving sheet is very high. For this reason, in printer mechanisms having a compact design in recent years, when the protective layer transfer sheet and the image-receiving sheet are carried, the protective layer transfer sheet remains contacted with the image-receiving sheet and is less likely to be separated from the image-receiving sheet. This causes jamming, or cockling or other phenomena at the time of printing due to poor slip properties.
This phenomenon will be described in conjunction with a printer conceptually shown in FIG. 4. The printer has a mechanism such that rollers 9 provided just before a platen roller 7 and a thermal head 8 for the reasons of a restriction on space sandwiches and holds therebetween an assembly of a protective layer transfer sheet 10 and an image-receiving sheet 11, with an image formed thereon, stacked on top of each other, and carries and feeds the assembly into between the platen roller 7 and the thermal head 8. Upon the completion of transfer of a protective layer from the protective layer transfer sheet 10, the protective layer transfer sheet 10 and the image-receiving sheet 11 are carried to respective stand-by positions for next transfer. In this case, the rollers 9 come apart from each other and is brought to a released state. This permits the protective layer transfer sheet 10 and the image-receiving sheet 11 to come apart from each other and to be carried to respective stand-by positions. In this operation, the extent of carrying of the protective layer transfer sheet 10 is different from the extent of carrying of the image-receiving sheet 11, and when they are carried in such a state that the separation of the protective layer transfer sheet 10 from the image-receiving sheet 11 is unsatisfactory, the protective layer transfer sheet 10, which is generally inferior in nerve to the image-receiving sheet 11, is unfavorably carried with the protective layer transfer sheet 10 attached to the image-receiving sheet 11.
In large printers, meandering or cockling occurs due to poor slip properties between the protective layer transfer sheet and the image-receiving sheet. In particular, when the film is slantingly fed due to poor accuracy of mounting of the cassette and the film, the occurrence of meandering or cockling is significant.